In many branches of industry polluting waste liquids are produced which cannot be drained as such in a sewer or watercourse, and generally it is to be avoided that this waste reaches the groundwater. Before being drained the waste liquid should, therefore, be stripped of noxious components as well as possible, e.g. by sedimentation or flotation, if necessary with addition of separation promoting agents, by biological purification, and the like.
For this purpose different kinds of purification devices are known which, in general, operate in such a manner that the purification effect is better as the residence time in such a device is longer.
It can happen, however, that the liquid supply increases in such a manner that the purification device is not able to operate in the required way, e.g. when, because of a failure in a duct or container, heavy rains or the influx of extinguishing water in case of fire, an excessive supply occurs which cannot be processed in the required way by the purification device.
Such an excessive supply can be many times the normal liquid flow rate, but the probability of its occurrence is very low. It is, therefore, not justified to design the purification device for such an excessive flow rate, but nevertheless provisions should be made for preventing a substantial pollution by the drained liquid in the occurence of such an exceptional condition. In some cases it is possible that at such an excessive supply the degree of dilution of the impurities increases so that, as such, a direct draining in the normal discharge means would not be objectionable, but, in that case, the purification device would be disturbed by the strong flow, e.g. in the case of biological purification when the danger exists that the active silt is entrained by the strong liquid flow, and then the purification device will become inoperative, and its recovery will require some time after the normal conditions have been restored again.
The usual purification devices having an inlet and an outlet and intermediate means for effecting the separation of impurities from a liquid are, therefore, often provided with an emergency outlet which is able to divert, in such circumstances, a very considerable part of the excess supply either towards a buffer vessel or directly towards a normal discharge duct, and in the former case the contents of the buffer vessel can be subjected to a purification treatment again after the emergency situation has ended. Such an emergency outlet can communicate, by means of an emergency overflow weir, with the inlet end of the purification device, which overflow weir is so much higher than the normal liquid level as corresponds to a level rise as a consequence of the excess supply.
Now the flow resistance of an overflow weir is mainly determined by the border friction and similar boundary effects, so that, as the thickness of the overflowing liquid layer increases, this resistance will increase gradually at a lower rate, which has the consequence that, at an increasing flow rate, the level rise will become smaller. As the length of the overflow weir is larger, the smaller will be the level rise as a consequence of an increasing supply, so that at an increasing length of the weir its characteristic curve will become flatter. Thus for obtaining a favourable emergency discharge a large length of the emergency overflow weir will be required in order to limit the level rise at an excessive supply, since a level rise will also lead to an increasing flow rate through the purification device which should even be avoided.
Generally an overflow weir is present at the outlet end of a purification device determining the liquid level in this device. When a separated impurity will float on the liquid, it is desired to keep the liquid level as constant as possible so as to ensure, under all circumstances, a good discharge of the floating layer by means of an open collecting trough or the like without mixing the carrier liquid, so that it is desired to use an outlet overflow weir with a relatively flat flow characteristic, or at least to operate in a relatively flat part of its characteristic. However this is unfavorable in the said emergency cases, since, as a consequence of this flat characteristic of the outlet overflow weir, a considerable increase of the flow rate will already take place at a relatively small level rise, unless the length of the emergency overflow weir would be made very large.
A draw-back of a large length of the emergency overflow weir is that such a weir requires much space and material which will lead to considerably higher cost, and, as said before, the probability that such an emergency overflow weir will have to--become operative, is relatively small.